The present invention relates to a method and apparatus for cooling a thermoplastic shape. More specifically, the present invention relates to a method of continuously cooling a thermoplastic shape having two open sides.
It is well known in the art to form thermoplastic pipe by drawing or extruding molten thermoplastic through an annular space formed between an inner male element or mandrel and an outer female element, bushing or sizing ring, while simultaneously cooling the thermoplastic to solidify the same. While dies of this type accurately form the inner and outer surfaces of the pipe and form a controlled uniform thickness, even though the thermoplastic may have a tendency to warp and shrink, there are numerous disadvantages to their use. A major disadvantage is that the dies are extremely complex and expensive. Since a given die generally forms only one size and thickness of pipe, in many cases the dies are shipped from one plant to another in order to avoid the expense of buying duplicate dies for each manufacturing facility. An attendant problem, again leading to complexity and expense, is the necessity of providing uniform cooling of the thermoplastic. Finally, since pipe is usually utilized for transporting fluids under pressure, it must have specified and uniform pressure rating and, hence, the wall thickness is maintained within a close tolerance, consequently, the dies utilized are incapable of producing pipe which varies in thickness along its length.
Since the drawing or extrusion of thermoplastic shapes other than pipe and having two open sides, such as channel shapes, is less widely practiced than the formation of pipe, little attention has been directed to improvements in the forming devices, even though the formation of such differing shapes creates numerous additional problems not encountered in the formation of pipe. Consequently, in many instances dies of the same character as those utilized in the formation of pipe are also utilized in the formation of thermoplastic shaped items having two open sides. Obviously, the wide variety of shapes with two open sides which can be and are formed greatly exaggerates the problem of utilizing complex and expensive forming equipment. In addition, there are times when it is desired to form a single elongated thermoplastic shape whose thickness varies along its length. This, of course, is impossile when utilizine pipe-type dies which surround the entire cross section of the shaped item. The desire to form an item which varies in thickness along its length also exaggerates the problem of cooling, since changing to a different thickness obviously requires changing the degree of cooling. Accordingly, there have been some attempts to simplify dies or means for forming thermoplastic shapes having two open sides. However, while these attempts have either completely or partially solved some of the above-mentioned problems, they have by no means solved all problems mentioned. Further, these more simplified methods are generally incapable of forming thermoplastic shapes having two open sides from thermoplastics which have a tendency to warp or shrink.
It is, therefore, an object of the present invention to overcome the above-mentioned and other problems of the prior art. Another object of the present invention is to provide an improved method and apparatus for continuously forming thermoplastic shapes having two open sides. Another further object of the present invention is to provide an improved method and apparatus for continuously forming thermoplastic shapes having two open sides which are inexpensive both as to method and equipment. A further object of the present invention is to provide an improved method and apparatus for forming thermoplastic shapes having two open sides which involves the utilization of improved cooling means. A still further object of the invention is to provide an improved method and apparatus for forming thermoplastic shapes having two open sides which can be varied in thickness along their length. Another and further object of the present invention is to provide a method and apparatus for forming thermoplastic shapes, having two open sides and which vary in thickness along their length, in which the cooling may be adjusted to compensate for the changes in cooling required by changes in thickness. Yet another object of the present invention is to provide an improved method and apparatus capable of accurately forming thermoplastic shapes having two open sides from thermoplastics which have a tendency to warp and shirnk. These and other objects of the present invention will be apparent from the following description.